Barium sulfate is widely used as a white pigment and extender pigment, e.g. in paints, cosmetics and plastics as well as a shadow forming material in x-ray photography or glossing enhancer.
In most applications barium sulfate is needed in the form of a dispersible powder or as a stable colloid dispersion. Due to environmental reasons the main part of paints today is water-based. Thus barium sulfate has to be dispersible in water. To satisfy this demand, U.S. Pat. No. 2,597,384 describes the preparation of colloidal barium sulfate by the reaction of a water soluble sulfate salt with an aqueous barium gluconate solution. However, U.S. Pat. No. 2,597,384 leads to brownish coloured dispersions, which prevent the use as pigment in non-coloured or white applications. Another disadvantage is its high content of water-soluble, toxic barium gluconate.
U.S. Pat. No. 4,551,497 claims the addition of silicate ions to an aqueous slurry of barium sulfate to modify the barium sulfate surface to improve its dispersibility. The resulting particles exhibit a primary particle size of about 0.2 to 20 μm. Thus they are limited to applications where transparency is not required.
To provide smaller particles, it is common practice to mill down natural barium sulfate to an average particle diameter of at least 100 nm. However, these particles can only be dispersed by adding great amounts of dispersants and show no specific benefits, that are expected for nano particles, e.g. narrow size distribution, high uniformity and crystallinity, transparency, etc. DE 10,229,289 offers a method to produce barium sulfate particles with a size of 0.1 to 2 μm. The d50 value can be steered in this range by varying the process parameters.
A method for synthesising barium sulfate nano particles with a primary particle size below 100 nm is offered in JP 58,120,520. The barium sulfate particles are prepared in aqueous solution in presence of a water soluble silicate salt to deposit barium silicate on the barium sulfate particles and to inhibit the growth of grains. The resulting product is obtained in form of agglomerates that have to be milled down to their primary particle size. This process leads to additional energy costs in addition to a contamination of the barium sulfate due to abrasion.
A similar primary particle sized barium sulfate is described in US 2003/0124048 A1. The inventors describe a process for producing barium sulfate with a grain size below 100 nm. Furthermore they disclose a method to increase the dispersibility of the barium sulfate by adding dispersing agents or wetting agents during or after the precipitation. Due to interactions of these additives with application media and additional costs, it is however desirable to provide barium sulfate that is dispersible without any additives.
This need exists also for other metal sulfates of commercial interest.
Therefore, an object of this invention is to obviate the above difficulties and to provide a method of producing nanoscale metal sulfates, e.g. barium sulfate, that are dispersible in polar media and solvents, preferably at least in water without the aid of any auxiliary substances, such as disperging agents or specific dispersing techniques.
One further object of the present invention is to provide a method of producing nanoscale metal sulfates, e.g. barium sulfate, that show an excellent degree of whiteness.
According to one preferred embodiment of the invention, a method is to be provided that furnishes nanoscale metal sulfate particles that are at least dispersible in water and methanol without the aid of any auxiliary substances, such as disperging agents and specific dispersing techniques.